1. Field of the Invention
The present invention relates generally to a camera lens system for image pickup devices, which is mounted in a mobile phone or personal digital assistant, and more particularly to a camera lens system for image pickup devices, which has a minimized overall length thereof to implement a micro lens, and in which lenses are formed to have suitable aspheric coefficients to reduce an incident angle of light on the lenses and improve the resolving power of the camera lens system.
2. Description of the Related Art
Recently, with the development of wireless communication technologies, there are many cases where digital cameras are mounted in mobile phones or Personal Digital Assistants (PDAs).
Generally, digital cameras are image recording media capable of photographing a plurality of still images without using films. Such a digital camera uses an image pickup device, which is a kind of semiconductor device, such as a Charged Coupled Device (CCD) or Complementary Metal Oxide Semiconductor (CMOS). In the digital camera, an object image formed on the image pickup device through a lens is converted into an electrical signal by the image pickup device, and the electrical signal is stored as a digital signal in a mobile phone or PDA in which the digital camera is mounted.
Since the digital camera for image pickup devices must be mounted in small mobile phones or PDAs, technology of miniaturizing the digital camera is a key point. In the prior art, an externally mounted digital camera as well as an internally mounted digital camera is also used.
The externally mounted digital camera is mounted at a portion of a body of a mobile phone to photograph an object in front or back of a Liquid Crystal Display (LCD) window of the mobile phone while being manually rotated by a user.
However, such an externally mounted digital camera is problematic in that it is inconvenient to use because a user always carries both a mobile phone and the digital camera, and uses the digital camera by attaching it to the mobile phone if necessary, and the total volume of the devices increases.
Therefore, a mobile phone with an internally mounted camera is generally used. In this case, the camera is installed at a hinge of the mobile phone that connects a cover part to a body.
As described above, the internally mounted camera must be mounted in a limited space, so the minimizing the size and weight of the camera is required.
FIG. 1 illustrates a conventional camera lens system for CCDs. The camera lens system for CCDs comprises a meniscus-shaped first lens 11 having a negative refractive index, a second lens 13 having a positive refractive index, a third lens 16 having a positive refractive index, a fourth lens 18 having a negative refractive index, and a CCD 12 for converting a light beam into an electrical signal.
An iris 14 is disposed between the first and second lenses 11 and 13 to adjust the quantity of light, and an optical filter 19 is used to block light rays, except for a required visible ray.
However, since the above lens system for image pickup devices requires at least four lenses to correct aberrations (especially, distortion aberrations), the number of lenses is increased and, therefore, the overall length of the lens system is increased. Thus, it is difficult to miniaturize the lens system.
Therefore, in order to solve this problem, a miniaturized lens system in which the number of lenses is decreased and, therefore, the overall length of the lens system is decreased is disclosed in Korean Patent Application No. 2001-25317. Referring to FIG. 2, the lens system comprises a meniscus-shaped first lens unit 21 having a convex incident surface facing an object, a meniscus-shaped second lens unit 23 on which a light beam is incident from the first lens unit 21 and which has a concave incident surface facing the object, an iris 22 disposed between the first and second lens units 21 and 23 to adjust the quantity of light, and an optical filter 24 disposed between the second lens unit 23 and an image-formation surface formed by the first and second lens units 21 and 23.
The first lens unit 21 is comprised of meniscus-shaped lens surfaces S1 and S2 (radius of curvature) which are convex toward the object, and the iris 22 is positioned between the first and second lens units 21 and 23 to adjust the quantity of light. The second lens unit 23 is comprised of lens surfaces S3 and S4 (radius of curvature) which are concave toward the object. The optical filter 24 is disposed at the back of the second lens unit 23 to block light rays except for a required light ray. Further, in the lens system, an image sensor 25 is provided to convert a light beam passing through the optical filter 24 into an electrical signal.
Such a conventional lens system is characterized in that the iris 22 is disposed between the first and second lens units 21 and 23 to precisely correct distortion aberrations by a symmetrical structure of the lens units 21 and 23 formed around the iris 22 and reduce the overall length of the lens system for image pickup devices, thus miniaturizing the lens system.
The first and second lens units 21 and 23 are formed in an aspheric shape, wherein the shapes of aspheric surfaces forming the first and second lens units 21 and 23 are expressed by the following Equation, assuming that a Z axis is taken as an optic axis, an X axis is taken as an axis perpendicular to the optic axis, light is directed forward, and K, A, B, C, D and E are aspheric coefficients,                     Z        =                                                            h                2                            r                                      1              +                                                1                  -                                                            (                                              1                        +                        K                                            )                                        ×                                                                  h                        2                                                                    r                        2                                                                                                                          +                      A            ×                          h              4                                +                      B            ×                          h              6                                +                      C            ×                          h              8                                +                      D            ×                          h              10                                +                      E            ×                          h              12                                                          [        1        ]            where r is the radius of curvature and h is an X axis length.
However, even though the conventional lens system uses aspheric surfaces, an incident angle of a light beam on an edge portion art of an image-formation surface of the image sensor 25 is excessively large, thus deteriorating the resolving power of the edge portion of the image-formation surface and causing distortion.
That is, in the conventional lens system, a light beam incident on a center portion of the image-formation surface of the image sensor 25 has a small incident angle according to the normal progress of the light beam, while a light beam incident on the edge portion thereof has a large incident angle to deteriorate the resolving power of the edge portion and cause distortion.
Generally, the image sensor 25 senses light from pixels formed on the image-formation surface, and the sensitivities of the pixels in the image sensor 25 are uniform, so the quantity of light incident on the image sensor 25 must be uniform at any position on the image sensor 25. However, the conventional lens system is problematic in that a difference between the incident angles of light beams on the center portion and the edge portion, respectively, is great, so there is a difference between quantities of light incident on the image sensor 25, thus causing the edge portion to be relatively dark and causing distortion.
Further, the conventional lens system is problematic in that its overall optical length is large to such an extent that it is equal to or greater than 5.3 mm, so it is difficult to miniaturize a mobile phone or PDA to which the lens system will be mounted.